Electrode for evolution of gaseous products and method of manufacturing thereof

ABSTRACT

The invention relates to an electrode suitable as anode for evolution of gaseous products comprising a metal substrate coated with at least one titanium suboxide layer having an interconnected porosity and containing catalytic noble metal oxides. The invention further relates to a method of manufacturing such electrode comprising applying a mixture of titanium suboxides and noble metal oxide-based catalyst on a valve metal substrate via cold gas spray technique.

FIELD OF THE INVENTION

The invention relates to an electrode suitable for functioning as anode in electrolysis cells, for instance as oxygen-evolving anode in electrolysis cells used in electrometallurgical processes, as chlorine-evolving anode either in chlor-alkali cells or as anode for hypochlorite generation in undivided cells.

BACKGROUND OF THE INVENTION

Substoichiometric compositions of titanium oxides of formula Ti_(x)O_(2x-1), with x ranging from 4 to 10, also known as titanium Magneli phases, are obtained by high temperature reduction of titanium dioxide under a hydrogen atmosphere. These suboxides are corrosion-resistant ceramic materials comparable to graphite in terms of electrical conductivity. In light of such corrosion resistance and conductivity characteristics these materials, which are produced both in massive and in powder form, may be used as protective coatings of metal substrates for electrochemical applications. There is also known the possibility of adding small amounts of doping agents to these ceramic materials, such as for instance tin oxide, in order to slightly increase their conductivity, stability and resistance to corrosion. In general, the deposition of these ceramic materials as metal substrate protectors is carried out starting from the material in powder form in accordance with known techniques, such as, hot flame spraying, plasma spraying or detonation thermal spraying. All of these techniques share the common feature of requiring a high operative temperature (>400° C.) in order to obtain an acceptable adhesion between sprayed powder particles and metal substrate. Furthermore, the good adhesion of deposited powder particles to the substrate also depends on the reciprocal nature of the substrate and the powder.

The above mentioned spraying techniques allow depositing very compact layers of ceramic material on the surface of a metal substrate. Such compactness is in fact required for an efficient anticorrosion function. More precisely, it is generally accepted in the art that the apparent density of the deposited ceramic layer must not be lower than 95% of the overall theoretical density in order to obtain an efficient material.

These ceramic materials may also be used as catalyst supports. In the manufacturing of an electrode starting from a metal substrate, the catalyst is applied in a step subsequent to the deposition of the titanium Magneli phase onto such substrate, generally by thermal decomposition of precursors. This mode of application, however, has the drawback of leading to the formation of ceramic layers wherein a major fraction of the catalyst applied turns out to be scarcely accessible to the electrolyte, the final product thus being hardly efficient in terms of activity and lifetime. Usually, in order to obtain electrodes of suitable performances for an industrial application, the loading of the Magneli phase-supported catalyst must be not lower than 20-30 g/m².

Moreover, the use of the above mentioned powder deposition techniques on metal substrates is not advisable whenever such powders also comprise noble metal oxides as catalysts, because such oxides are not stable to temperatures above 400° C. and tend to decompose, thereby hindering an appropriate deposition. The preparation of titanium suboxide and noble metal oxide mixtures to be subsequently deposited onto a substrate by means of the above mentioned techniques is hence not easy to practise.

The inventors surprisingly found out a method for manufacturing electrodes comprising a valve metal-based substrate coated with at least one layer of noble metals or oxides thereof supported on titanium suboxides overcoming the inconveniences of the prior art.

SUMMARY OF THE INVENTION

Various aspects of the invention are set out in the accompanying claims.

Under one aspect, the invention relates to an electrode for evolution of gaseous products in electrolytic cells comprising a valve metal substrate whereto at least one layer of a coating having an interconnected porosity is attached, the layer consisting of at least one catalyst containing noble metals or oxides thereof taken alone or in admixture, supported on titanium suboxide species expressed by the formula Ti_(x)O_(2x-1), with x ranging from 4 to 10, the specific catalyst loading being comprised between 0.1 and 25 g/m².

The term interconnected is used herein to mean a porosity mostly consisting of a network of pores in mutual fluid communication and not isolated. In order to obtain a layer having an interconnected porosity, it is normally considered that the apparent density of such layer must be lower than 95% of the overall theoretical density which a compact layer with no porosity at all having an equivalent composition would exhibit.

Under another aspect, the invention relates to an electrode for evolution of gaseous products in electrolytic cells consisting of a valve metal substrate and at least a coating layer having an interconnected porosity bound thereto, said at least one layer comprising at least one catalyst consisting of noble metals or oxides thereof taken alone or in admixture, supported on a mixture of titanium suboxides of formula Ti_(x)O_(2x-1), with x ranging from 4 to 10, said at least one layer being deposited onto said substrate by cold gas spray technique. The term cold gas spray is used herein to mean a deposition technique of solid particles onto substrates supposedly known to a person skilled in the art, based on accelerating powder particles transported by a compressed carrier gas. During their trajectory, the carrier gas and the particles are split into two different paths so that the time of residence of powders inside the hot gas phase is limited, thereby preventing powders to be heated above 200° C.

The inventors have surprisingly observed that the deposition via cold gas spray technique of a Magneli phase-type ceramic powder, for example consisting of a titanium Magneli phase powder previously catalysed with noble metal oxides by thermal decomposition of precursors, onto a substrate made of a valve metal such as titanium, tantalum, zirconium or niobium, leads to a structure of surprisingly enhanced duration even at very low catalyst loadings. In particular, the lifetime of an electrode obtained as hereinbefore described in common industrial electrochemical applications is much higher compared to the one of an electrode having the same nominal content of catalyst but prepared by traditional thermal decomposition.

In one embodiment, the valve metal of choice for the substrate is titanium.

In one particular embodiment, the coating layer has an interconnected porosity with an apparent density ranging higher than 75% and lower than 95% of the overall theoretical density.

In another embodiment, the electrode has a coating layer containing a specific catalyst loading of 0.1 to 10 g/m².

In yet another embodiment, the noble metal oxide-based catalyst consists of iridium oxide.

Under another aspect, the invention relates to a method for manufacturing an electrode according to the invention comprising the steps of: preparing a titanium suboxide powder expressed by the formula Ti_(x)O_(2x-1), with x ranging between 4 and 10; impregnating said powder with a precursor solution of a noble metal oxide-based catalyst with subsequent thermal decomposition; depositing the obtained powder on a valve metal substrate by cold gas spray technique.

Under yet another aspect, the invention relates to an electrolysis cell comprising a cathodic compartment containing a cathode and an anodic compartment containing an anode, wherein said anode of said anodic compartment is an electrode as hereinbefore described.

Under yet another aspect the invention relates to an industrial electrochemical process comprising the anodic evolution of a gas from an electrolytic bath on an electrode as hereinbefore described.

The following examples are included to demonstrate particular embodiments of the invention, whose practicability has been largely verified in the claimed range of values. It should be appreciated by those of skill in the art that the compositions and techniques disclosed in the examples which follow represent compositions and techniques discovered by the inventors to function well in the practice of the invention; however, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the scope of the invention.

EXAMPLE 1

An appropriate volume of titanium Magneli phase powder in admixture with iridium oxide was sprayed onto a titanium grade 1 sheet of 10 cm×10 cm×0.2 cm size, previously sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in order to obtain a rough surface free of titanium oxide species. Such powder was obtained by mixing a suitable mass of titanium Magneli phase powder previously sieved to a size range of 100 to 400 μm to an acidic solution containing a soluble precursor of iridium, namely iridium trichloride in aqueous HCl. Such mixture was then calcined in oxidising atmosphere in a rotary oven.

The spraying parameters selected for cold gas spray technique application were the following:

-   Nozzle-to-sheet gap: 20 mm -   Primary gas: nitrogen -   (Primary) gas pressure: 30 bar -   Gas flow-rate: 6 m³/h -   Feeder gas flow-rate: 4% -   Throat size: 1 mm -   Scan rate: 50 mm/s

As a final target of the cold gas spraying process, a homogeneous coating containing 10 g/m² of iridium was obtained.

The thus obtained electrode was identified as sample #1.

EXAMPLE 2

An appropriate volume of titanium Magneli phase powder in admixture with ruthenium oxide was sprayed onto a titanium grade 1 sheet of 10 cm×10 cm×0.2 cm size, previously sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in order to obtain a rough surface free of titanium oxide species. Such powder was obtained by mixing a suitable mass of titanium Magneli phase powder previously sieved to a size range of 100 to 400 μm to an acidic solution containing a soluble precursor of ruthenium, namely ruthenium trichloride in aqueous HCl. Such mixture was then calcined in oxidising atmosphere in a rotary oven.

The spraying parameters selected for cold gas spray technique application were the following:

-   Nozzle-to-sheet gap: 20 mm -   Primary gas: nitrogen -   (Primary) gas pressure: 30 bar -   Gas flow-rate: 6 m³/h -   Feeder gas flow-rate: 4% -   Throat size: 1 mm -   Scan rate: 50 mm/s

As a final target of the cold gas spraying process, a homogeneous coating containing 10 g/m² of ruthenium was obtained.

The thus obtained electrode was identified as sample #2.

COUNTER EXAMPLE 1

An appropriate volume of titanium Magneli phase powder in admixture with iridium oxide was plasma-sprayed onto a titanium grade 1 sheet of 10 cm×10 cm×0.2 cm size, previously sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in order to obtain a rough surface free of titanium oxide species. Such powder was obtained by mixing a suitable mass of titanium Magneli phase powder previously sieved to a size range of 100 to 400 pm to an acidic solution containing a soluble precursor of iridium, namely iridium trichloride in aqueous HCl. Such mixture was then calcined in oxidising atmosphere in a rotary oven.

The following spraying parameters were applied:

-   Nozzle-to-sheet gap: 90 mm -   Primary gas: argon -   (Primary) gas pressure: 60 bar -   Throat size: 5 mm -   Scan rate: 200 mm/s

As a final target of the plasma-spraying process, a homogeneous coating containing 10 g/m² of iridium was obtained.

Due to the high temperature reached by the powder during the plasma spraying process, it was observed that Magneli phase-supported iridium oxide was partially converted to iridium metal.

The thus obtained electrode was identified as sample #C1.

COUNTER EXAMPLE 2

An appropriate volume of titanium Magneli phase powder, previously sieved to a size range of 100 to 400 μm, was plasma-sprayed onto a titanium grade 1 sheet of 10 cm×10 cm×0.2 cm size, previously sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in order to obtain a rough surface free of titanium oxide species.

The following spraying parameters were applied:

-   Nozzle-to-sheet gap: 90 mm -   Primary gas: argon -   (Primary) gas pressure: 60 bar -   Throat size: 5 mm -   Scan rate: 200 mm/s

An acidic solution was subsequently prepared containing ruthenium trichloride and iridium trichloride in suitable concentration and stoichiometric ratio. The above plasma-sprayed titanium sheet was dipped in such solution for 15 seconds, allowed to dry slowly and finally placed in a batch furnace at 450° C. in oxidising atmosphere. In order to obtain the required noble metal loading (5 g Ru/m² and 2 g Ir/m²) the dipping and thermal decomposition cycle was repeated 4 times.

The thus obtained electrode was identified as sample #02.

COUNTEREXAMPLE 3

A known volume of acidic solution containing a soluble precursor of ruthenium, namely highly concentrated ruthenium trichloride, was applied by electrostatic spraying onto a titanium grade 1 sheet of 10 cm×10 cm×0.2 cm size, previously sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in order to obtain a rough surface free of titanium oxide species. The solution was allowed to dry slowly and then decomposed in a batch furnace at 450° C. in oxidising atmosphere.

In order to obtain the required noble metal loading (24 g Ru/m²) the electrostatic spraying and thermal decomposition cycle was repeated 18 times.

The thus obtained electrode was identified as sample #03.

The samples obtained in the above examples and counterexamples were subjected to electrolysis tests, as reported in Table 1 below:

TABLE 1 Hypochlorite Service life production in faradaic accelerated Catalyst efficiency* test** loading Sample ID Resistivity (Ωm) (%) (hours) (g_(NM)/m²) 1 5 exp −6 73 1600 10 (Ir) 2 5 exp −6 75 1550 10 (Ru) C1 5 exp −3 39 240 10 (Ir) C2 5 exp −6 71 290 5 + 2 (Ru + Ir) C3 5 exp −6 78 150 24 (Ru) *Hypochlorite production faradaic efficiency: measure of faradaic efficiency by titration of active chlorine present in an electrolyte sample obtained starting from an aqueous NaCl solution at 30 g/l subjected to electrolysis for 10 minutes, at 25° C. and at a current density of 2 kA/m². The sample under test is the working anode, while the counterelectrode consists of a titanium sheet. **Accelerated test: electrolysis carried out in a solution of 5 g/l NaCl and 50 g/l Na₂SO₄, 30° C., 1 kA/m². Anode and cathode are made of the same material. Electrode polarity is reversed every 2 minutes.

The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

Throughout the description and claims of the present application, the term “comprise” and variations thereof such as “comprising” and “comprises” are not intended to exclude the presence of other elements, components or additional process steps.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application. 

1. An electrode for evolution of gaseous products in electrolytic cells comprising a valve metal substrate whereto at least one layer of a coating having an interconnected porosity is attached, said at least one layer being comprised of titanium suboxides expressed by the formula Ti_(x)O_(2x-1), with x ranging between 4 and 10, in admixture with at least one catalyst based on noble metals or oxides thereof, the specific catalyst loading being comprised between 0.1 and 25 g/m².
 2. An electrode for evolution of gaseous products in electrolytic cells comprising a valve metal substrate whereto at least one layer of a coating having an interconnected porosity is attached, said at least one layer being comprised of titanium suboxides expressed by the formula Ti_(x)O_(2x-1), with x ranging from 4 to 10, in admixture with at least one catalyst based on noble metals or oxides thereof, said at least one layer being deposited on said substrate by cold gas spray technique.
 3. The electrode according to claim 1 wherein said valve metal of said substrate is titanium.
 4. The electrode according to claim 1 wherein said at least one coating layer attached to the substrate has an apparent density of 75 to 95% of the overall theoretical density of said layer.
 5. The electrode according to claim 1 wherein the specific catalyst loading in least one coating layer ranges between 0.1 and 10 g/m².
 6. The electrode according to claim 1 wherein said at least one catalyst based on noble metal oxides consists of iridium oxide.
 7. A method for manufacturing an electrode according to claim 1 comprising the following steps: preparing of a powder of titanium suboxides expressed by the formula Ti_(x)O_(2x-1), with x ranging between 4 and 10, impregnating impregnation of said powder with a precursor solution of a noble metal or noble metal oxide-based catalyst carrying out thermal decomposition, and carrying out deposition of said powder on a valve metal substrate by cold gas spray technique.
 8. The electrolysis cell comprising a cathodic compartment containing a cathode and an anodic compartment containing an anode, wherein said anode of said anodic compartment is an electrode according to claim
 1. 9. The industrial electrochemical process comprising carrying out anodic evolution of a gas on an electrode according to claim 1 from an electrolytic bath. 